Flying Spiders: Effects of the Dragline Length and the Spider Mass in Free-Fall

Many species of spiders move from one location to another using a remarkable aerial dispersal “ballooning”. By ballooning, spiders can reach distances as far as 3200 km and heights of up to 5 km. Though a large number of observations of spider ballooning have been reported, it remains a mysterious phenomenon due to the limited scientific observation of spider ballooning in the field, high uncertainties of the meteorological conditions and insufficient controlled laboratory experiments. Most of the ballooning spiders are spiderlings and spiders under 3 mm in length and 0.2 to 2 mg in mass with a few exceptions of large spiders (over 3 mm in length, over 5 mg in mass). What physical mechanism dominates the three stages of spider ballooning — take-off, flight, and settling? Many factors have been identified to influence the physical mechanism, including a spider’s mass, morphology, posture, the silken dragline properties, and local meteorological conditions (e.g., turbulence level, temperature and humidity). A thorough understanding of the roles of key parameters is not only of ecological significance but also critical to advanced bio-inspired technologies of airborne robotic devices. This work aims to determine how the dragline length and spider mass affect the interaction of the spider-dragline system in the free-fall scenario. Experiments using a thread of different lengths and a sphere of different masses to mimic the spider-dragline were carried out. The first sets of tests focused on the spider-dragline system, rather than the fluid flow. High-speed images of a spider-dragline falling in a closed container of air were recorded with 1500 frames per second at Reynolds numbers of several thousand, based on the spider dragline and the local relative velocity. Image data allow for tracking the vertical velocities and acceleration of the spider-dragline, as well as the drag force acting on the spider-dragline. Terminal velocities in the settling stage are compared with estimates using various fluid dynamics models in previous work. Such results under controlled laboratory conditions are expected to shed lights on the intriguing flow physics of spider ballooning at the settling stage and to inform future experiments and numerical models.